I would love to see it in the main game, but not just as a throw in.
Add different kingdoms with different trainers (like the one in Zandar yet different) where you can learn that culture's fighting styles. A knightly squire cant just pick up a katana or naginata and go to town, the stances, swings, cutting styles etc are all different.
Say you went to a barbarian style town and learned a straight forward all out offensive style with berzerker like swings and lower defense but higher damage. Maybe a medieval english culture and learned a balanced offense/defense style that was more armor reliant. The Japanese styles could be elegant and exotic style swings, high damage with slightly better ability to block with a weapon, but as a balance you couldnt fight well, or your attack speed and damage were severely hampered if you wore anything over light- medium armor. Cartain cultures could fight better from horseback and gain benefits while fighting mounted while suffering penalties on foot.
This could really be expanded on with the various factions and nations involved since maybe one nation doesnt like another, and your stand with one affects how thier allies view you also, so you may have to pick a faction where say, merchants pay less and certain weaponry is much more expensive in order to learn thier fighting styles which may be good to compinsate for the hard road of moneymaking.
As far as Japanese weapons not being able to pierce armor, thats pretty far from the truth. The Japanese lamellar was no joke and thier weapons were quite effective. Thier weapons were really no different than western weaponry as far as functionality. Sure a Katana might have a rough time getting through plate, but its slender enough to go between plates, the same as fighting an armored opponent with a long sword. A Nodachi would do just fine against armor.
Certain other styles would be nice too, middle eastern fighting styles and weaponry, russian, chinese. The bigger the world the better and the bigger this already great game gets the better. More variety = more play value in the long run since more cultures = more options and replayability.